The present invention relates generally to radio signal broadcast apparatus, and particularly to subcarrier signal generator apparatus.
Some FM radio signal broadcasts include a subcarrier signal. The subcarrier signal delivers, for example, paging data in implementation of a paging system transmitting paging messages to a population of paging devices each monitoring the subcarrier signal. The paging system thereby uses the existing FM signal broadcast facility. The FM broadcast facility receives digital paging messages from a paging system through, for example, a high speed modem coupling the FM broadcast facility and a paging system clearing house. The FM broadcast facility includes a subcarrier generator. The subcarrier generator receives the digital paging message information and incorporates this paging message information into its output, a subcarrier signal. The subcarrier generator applies this output to an exciter, and subsequent FM signal broadcasting devices to provide paging messages in the subcarrier portion of the transmitted FM radio signal.
The quality of a subcarrier signal degrades in the analog circuitry of the typical subcarrier generator device. Frequency-dependent attributes of analog devices, e.g., amplifiers and lowpass filters at the output stage of the subcarrier generator device, convolve a frequency-dependent non-linear phase shift with the generated signal. This adds distortion to the subcarrier signal and degrades overall quality of the information provided by subcarrier signal.
A typical subcarrier generator receives a digital data stream at a symbol generator and applies a resulting waveform symbol sequence to a bandpass finite impulse response (FIR) filter. The FIR filter properly shapes the waveform symbol sequence to achieve ultimately a desired shape and reference frequency, i.e., center frequency, in the transmitted subcarrier signal. A set of the FIR filter coefficients define the resulting shape and therefore the center frequency of the subcarrier signal. Thus, in a typical subcarrier signal generator the FIR coefficients are established at the time of design according to a desired subcarrier signal shape and center frequency. In other words, an ideal shape and center frequency are selected and FIR coefficients calculated to obtain such shape and center frequency.
The FIR filter provides its output to a sequence of output stage devices, e.g., digital-to-analog converter, amplifier, lowpass filter, and final output stage amplifier. Unfortunately, the output stage devices introduce frequency-dependent amplitude and phase distortion in the subcarrier signal relative to the ideal subcarrier signal shape and center frequency. While such subcarrier generator devices reliably produce a useable subcarrier signal, there remains need and opportunity for improvement in the subcarrier signal generated. An analog filter, with good group delay correction, can compensate for the non-linear phase shift introduced, but such a filter is expensive and difficult to design and build and is physically large due to the delay correction section.
It would be desirable, therefore, to provide a subcarrier signal generator at relatively lesser expense, yet compensate for frequency-dependent characteristics and provide a high quality subcarrier signal.